The present invention pertains to a control system and more particularly, a power management control system which is preferably used for controlling the operation of heating, air-conditioning and ventilating units of the type normally used in a commercial building. In accordance with one feature of this invention the system is operated on a demand basis permitting only a predetermined number of stages to operate at any one time but permitting other demanding stages to operate in sequence on the basis of dropping the longest running stage and selecting in its place the longest waiting stage. In accordance with another aspect of this invention the system is preferably used for controlling the volume of outside air accepted by the units as a function of a continuously updated cumulative count of the occupants in the building at any one time.
The electric utility companies in many areas of the country, for commercial customers, base their rates on basically two factors. The rate is based primarily on the kilowatt hour usage of the customer and is also based upon peak demand which is resettable to zero either monthly or yearly depending on the policies of the power company of the customer. A further introduction recently of a fuel oil adjustment percentage factor adds substantially also to the overall costs. The demand charge can be substantial and may be assessed even though the customer operates at peak demand for only a very short period of time (possibly only 15 minutes) out of a total month or year of operation.
Accordingly, one object of the present invention is to provide a power management control system for controlling a plurality of stages such as heating, air-conditioning and ventilating units and maintaining the peak demand at an acceptable level by limiting the total number of stages that can operate at any one time.
One known system operates so that only a limited number of units are operating at one time. However, this known system is unsatisfactory in that it operates strictly on a time clock basis permitting certain units to operate only during a fixed time period. This sytem is unsatisfactory primarily because the units may not be operated over a sufficient period of time. For example, with this known system a unit may be turned on by its thermostat near the end of its pre-permitted time cycle and thus is almost immediately turned off at the end of its time cycle. This intermittent operation of the unit is not satisfactory and is not a mode in which the unit is meant to operate.
Accordingly, another object of the present invention is to provide a power management control system wherein any unit that is permitted to operate is operated either until it is satisfied or at least a sufficient predetermined time period.
Another object of the patent invention is to provide a power management control system wherein possibly other types of electrically operated units or machines that are permitted to operate are operated either until satisfied or at least until a sufficient predetermined time period has elapsed.
In commercial buildings and especially those which are open to the public a system cannot operate to simply recirculate air from within the building. There is a requirement that a predetermined volume of fresh air be introduced into the building via the heating, air-conditioning and ventilating unit usually 5 to 15 cubic feet of air per minute per person depending on the business conducted in the building. Usually each unit comprises a fan and an associated damper which may be operated to completely recycle the air in the building or introduce predetermined amounts of fresh air from outside. It is typical to design the system on the basis of a maximum possible occupancy, provide for the fresh air through the ventilating fan and set the dampers thereby. In some systems the dampers on all units are simply maintained at their minimum open setting at all times that the HVAC units are operating thereby continuously causing outside air to be brought into the unit. This is most wasteful in that if, for example, it is cold outside and the heating unit is operating then the constant introduction of the cooled outside air requires more energy to be expended in heating the inside of the building. This is especially wasteful when the occupancy in the building is low.
Accordingly, a further object of the present invention is to provide a power management control system that is sensitive to the cumulative total of occupants in a building. The system preferably controls the ventilating fans and dampers of each HVAC unit, starting the ventilating fans and opening or partially opening a limited number of these dampers as a function of the number of people in the building.
Still another object of the present invention is to provide a power management control system that is sensitive to an occupancy count in a building for controlling the operation of possibly other types of electrically operated units.
Still a further object of the present invention is to construct a system in accordance with the principals of this invention which is efficient in operation, relatively easy to maintain, and which provides for smooth operation of the units which the system controls.